1. Field of the Invention
The invention relates in general to a photolithography process, and more particularly to a phase-shifting mask and the manufacturing method thereof.
2. Description of the Related Art
In semiconductor fabrication, photolithography is an important and indispensable technique that is used to transfer circuit layout patterns through a mask onto predefined locations on a semiconductor wafer. Many processes in semiconductor fabrications, such as etching and ion implantation, require the use of photolithography. In a photolithographic process, resolution and depth of focus (DOF) are two major checkpoints used to appraise the quality of the pattern definition. A high level of integration requires a high resolution of pattern definition since the feature size is very small. To increase the resolution, a laser source with a very short wavelength, such as a krypton (Kr) deep ultraviolet laser with a wavelength of 2480 .ANG. (angstrom), is used as the exposure light in the photolithographic process. The use of a short-wavelength exposure light, however, will result in a shallow DOF. To allow high resolution and good DOF, one solution is to use the so-called phase-shifting mask (PSM).
Fundamentally, a PSM is formed by adding phase shifter layers to a conventional mask, which phase shifter layers cause destructive interference to the light passing through it such that the contrast and resolution of the resulting pattern definition can be increased. One benefit of the PSM is that it can increase the resolution of pattern definition without having to change the wavelength of the exposure light.
In semiconductor fabrication, line patterns are usually required to form structures such as metal interconnections, bit lines and word lines of dynamic random access memories (DRAM). Conventionally, a post-photoresist is used to execute a photolithography process. In highly integrated of devices, the line width of the line patterns is increased. The so-called alternating PSM is used to define line patterns with a high resolution.
A conventional alternating PSM photolithographic process is illustratively depicted in the following with reference to FIG. 1, which is a cross-sectional view of a conventional alternating PSM. A method of fabricating the conventional alternating PSM comprises the step of forming a patterned chromium film 102 as a mask on a light-penetrable substrate 100 such as a glass substrate or a quartz substrate, and the step of etching the substrate 100 to form a trench 103. There are two areas that can cause a phase shift of 180.degree. when incident light passes through them. These are the thickness 11 of a region 10 of the substrate 100 which is exposed by the chromium film 102 and the thickness 12 of a region 20 where the trench 13 is formed. However, the difference between the region 10 of the substrate 100 which is exposed by the chromium film 102 and the region 20 where the trench 13 is formed directly affects the phase shift caused to the incident light. Also, it is difficult to control the etching process that creates thickness 13. As a result, the incident light passing through the region 10 of the substrate 100, which is exposed by the chromium film 102, and the region 20 where the trench 13 is formed cannot undergo the phase shift of 180.degree.. This means that the alternating PSM can not achieve the required interference.
Another conventional alternating PSM photolithographic process is illustratively depicted in the following with reference to FIGS. 2A-2C. Referring first to FIG. 2A, a chromium film 202 is formed on a light-penetrable substrate 200 such as a glass substrate, a quartz substrate, or other suitable material. Next, referring to FIG. 2B, the chromium film 202 is defined and a chromium film 202a is left to cover a sheltered region S. A phase shift layer 204 is formed on the light-penetrable substrate 200. The material of the phase shift layer 204 is, for example, SiO.sub.x N.sub.y or Mo.sub.z SiO.sub.x N.sub.y. Then, referring to FIG. 2C, the phase shift layer 204 is defined by using the light-penetrable substrate 200 as an etching stop layer and a phase shift layer 204a is left to cover a shifting region P.
A shift angle from the phase shift layer 204 is the key to determining either destructive interference or constructive interference to light passing through a penetrating region T and the shifting region P during exposure. One factor that influences the shift angle is the characteristics of the phase shift layer 204. Another important factor is the thickness 205 of the phase shift layer 204. A conventional method of fabricating the mask described above is to define a pattern of the chromium film 202 and deposit and pattern the phase shift layer 204a on the semi-finished substrate 200, which has the chromium film 202 and a photoresist layer already formed thereon. It is difficult to gain a uniform thickness from coating a material on a square substrate. Furthermore, coating the phase shift layer 204 on the substrate 200 with a pattern of chromium film 202 already formed on it results in poor step-coverage. The reasons described above make the thickness 205 of the phase shift layer 204a non-uniform and make a difference in the phase angles of the incident light passing through regions of the mask. This results in the poor resolution of the PSM illustrated above.